Orcas and seals compete for a limited number of chinook salmon

It’s always been troubling to me that the Southern Resident
killer whales, which frequent Puget Sound, have struggled to
maintain their population, while other fish-eating resident orcas
seem to be doing much better.

Now several researchers have analyzed the energy needs of all
the seals, sea lions and killer whales that eat chinook salmon
along the West Coast, from California to Alaska. The study provides
a possible explanation, one that is consistent with what many
scientists have suspected all along. Here’s how I explained it in a
story written for the Encyclopedia
of Puget Sound:

“Puget Sound’s endangered killer whales are waiting at the
end of a long food line for a meal of chinook salmon — basically
the only food they really want to eat.

“Ahead of them in the line are hundreds of salmon-craving
killer whales in Alaska and British Columbia. Even farther ahead
are thousands of seals and sea lions that eat young chinook before
the fish have a chance to grow to a suitable size for
orcas.”

My story contains plenty of numbers to explain what this is all
about.

This issue of competition for food is not a simple one to
discuss or resolve. But the new paper, published in the journal
Scientific
Reports, adds an important perspective when trying to answer
the question: “Do we have too few salmon or too many marine
mammals?”

From a historical viewpoint, the answer must be that we have too
few salmon. But from a management perspective, we might have to
conclude that the ecosystem is out of balance and that we have been
restoring some marine mammal populations faster than we are
restoring the salmon that they eat.

In an intriguing study published in March in the journal
Nature Ecology & Evolution (PDF 840 kb), a group of West
Coast researchers investigated whether it is better to recover
populations of prey species first, followed by predator species, or
if it is better to recover predator species first, followed by prey
species.

Protecting predators first — which is usually the way humans do
things — may slow the growth of prey species or even trigger a
population decline, the report says. That creates a problem for
predators that specialize in that one kind of prey as well as for
those that have no access to alternative prey.

It may seem logical to rebuild the prey species first, the
authors say. But, with some exceptions, recovering prey species
first causes the combined predator and prey populations to peak at
high levels that are unsustainable in the overall ecosystem.

“In the real world,” the paper states, “transient dynamics like
these that result from eruptions of prey populations can lead to
surprising cascades of ecological interactions and complex but
often mismatched management responses.”

The authors conclude that the fastest way to restore depressed
populations is through synchronous recovery of predators and prey
by carefully rebuilding two or more populations at the same
time.

Management tactics may include culling predators even before
optimal population numbers are reached. Such actions require
careful study, as culling may produce unexpected consequences,
according to the report.

Other options include protecting multiple species within
protected geographic or marine areas or focusing on single species
by protecting select habitats or reducing human exploitation.

For Southern Resident killer whales, the question will be
whether populations of other marine mammals — particularly harbor
seals in Puget Sound —should be controlled. If so, how would people
go about doing that?

One related issue that needs more study is the effect that
transient killer whales are having on the Salish Sea population of
seals and sea lions. As the Southern Residents spend less time
searching for chinook salmon in the inland waterway, the
seal-eating transients are being spotted more and more by people
along the shores of Puget Sound.

Some studies estimate that the transients need an average of one
to two seals each day to maintain their energy needs, although we
know these whales also eat smaller sea otters and larger California
and Steller sea lions, as well as an occasional gray whale.

Are the transients culling the population of harbor seals in
Puget Sound or at least limiting their growth? Even before the
transients were showing up frequently, biologists were telling us
that the overall harbor seal population appeared to be peaking and
perhaps declining.

It would be interesting to create a future-looking computer
model that could account for populations of salmon and marine
mammals under various scenarios — including possible management
actions by humans and the ongoing predation by transient killer
whales.

If we want to keep things more natural while helping out the
Southern Residents, maybe somebody could come up with a strategy to
attract and maintain a healthy population of seal-eating transient
orcas within the Salish Sea.